This invention pertains to a method for remotely replacing deteriorated tube ends in a heat exchanger. It pertains particularly to a method for remotely removing the deteriorated end portion of an existing tube from a tubesheet of a heat exchanger and replacing it with a new tube aligned and welded to the end of the existing tubes and to the tubesheet.
In steam generators such as used for nuclear power plants, bundles of vertical U-shaped heat exchanger tubes are welded into a thick tubesheet located at the lower end of the heat exchanger. During operation of such steam generators over an extended period of time, such as 10 years or more, a corrosive sludge material forms and accumulates on the inner upper surface of the tubesheet. This accumulated sludge material causes corrosion of the tubes to an extent of potential or actual tube failure. To correct this serious problem without undesired replacement of the heat exchanger, it is necessary to remove the old deteriorated tube end portions from the tubesheet and replace them with new tube portions which are remotely positioned and pressure-tightly welded into place without removing and dismantling the entire heat exchanger assembly. However, because of the remote and relatively inaccessible location of the tubes and the radioactive environment which is usually involved, such tube replacement is very difficult to accomplish reliably and safely. Thus, a suitable solution to this tube replacement problem has been needed, particularly for heat exchangers used in the nuclear power industry.
Some portable equipment and procedures have been previously developed by industry for rotary trimming the ends of tubes. For example, U.S. Pat. No. 3,898,714 to McFadden discloses a pipe alignment clamp externally fitted around adjacent pipes to hold them in coaxial alignment prior to welding. U.S. No. 4,000,556 to Ciminero; U.S. No. 4,161,810 to Beard et al; and U.S. No. 4,393,564 to Martin disclose methods and apparatus for hydraulically extracting heat exchanger tubes from tubesheets. Also, U.S. No. 3,962,767 to Byerley et al shows repair of heat exchanger tubes by severing the tube, inserting a sleeve and internally welding the sleeve in place. U.S. No. 3,979,810 to Krips et al shows attachment of a tube into a plate opening by welding the tube end and also by hydraulic deforming and roll swaging the tube wall against the plate opening.
The prior art has apparently not provided apparatus and methods suitable for remotely removing and replacing deteriorated tube ends in heat exchangers, particularly those in relatively inaccessible locations and in radioactive environments which is usually involved where such tube end replacement is very difficult to accomplish reliably and safely. However, the present invention provides an advantageous and desirable solution to this tube end replacement problem by providing a method for remotely operated removal of deteriorated tube ends and their replacement with new tubes.